IPT technology is an area of increasing development and IPT systems are now utilised in a range of applications and with various configurations. Typically, a primary side (i.e., an inductive power transmitter) will include a transmitting coil or coils configured to generate an alternating magnetic field. This magnetic field induces an alternating current in the receiving coil or coils of a secondary side (i.e., an inductive power receiver). This induced current in the receiver can then be provided to some load, for example, for charging a battery or powering a portable device. In some instances, the transmitting coil(s) or the receiving coil(s) may be suitably connected with capacitors to create a resonant circuit. This can increase power throughput and efficiency at the corresponding resonant frequency.
A problem associated with IPT systems is regulating the amount of power provided to the load. It is important to regulate the power provided to the load to ensure the power is sufficient to meet the load's power demands. Similarly, it is important that the power provided to the load is not excessive, which may lead to inefficiencies. Generally, there are two approaches to power control in IPT systems: transmitter-side power control and receiver-side power control.
In transmitter-side power control, the transmitter is typically controlled to adjust the power of the generated magnetic field (for example, by adjusting the power supplied to the transmitting coil(s) or by adjusting the tuning of the transmitter).
In receiver-side power control, the receiver is controlled to adjust the power provided to the load from the receiving coils (for example, by including a regulating stage or by adjusting the tuning of the receiver).
A problem associated with some receiver-side power control systems that rely on regulating stages is that such regulating stages may need to include DC inductors. Such DC inductors can be relatively large in terms of volume. There may be demand to miniaturise receivers so that they may fit within portable electronic devices, so it may be desirable that the DC inductor be eliminated from the receiver circuitry.
Power control systems (whether they are transmitter-side or receiver-side) that adjust the tuning of the coils will typically include an arrangement of switches as part of the resonant circuit. These switches can be selectively activated to short or open parts of the resonant circuit, thus affecting the tuning of the resonant circuit and the power transmitted or received. However, since these switches are part of the resonant circuit, there can be resulting high losses due to peak currents or voltages associated with the switches. Further, the blocking voltage of these switches must be rated for the voltages required by the load. Large voltage switches can be expensive and difficult to miniaturise.
U.S. Pat. No. 6,705,441 discloses a resonant receiving coil with a series control switch. The control switch is selectively switched on and off to regulate the amount of power provided to the load. A problem associated with this approach (and as described above) is that the control switch must be rated for large voltages. Further, even if zero-current crossing is implemented to minimise transient currents through the switch as it is switched on and off, there can be undesirable large voltage spikes observed across the switch.
U.S. Pat. No. 6,705,441 further discloses a system for controlling the amount of power received by a non-resonant receiving coil by adjusting a coupled resonant pickup coil. Though this system eliminates the need for a DC inductor, it relies on a non-resonant receiving coil which may not be compliant with current and future consumer electronics industry standards for wireless power transfer, such as the Wireless Power Consortium Qi Standard. A problem associated with this system is that the resonant pickup coil will draw power from the transmitter and will affect the resonance of the transmitting coil. Further, since the resonant pickup coil receives transmitted power, the switch associated with the control of the resonant pickup coil may result in high losses.
Accordingly, embodiments may provide an improved coupled-coil power control for inductive power transfer systems or may at least provide the public with a useful choice.